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Assessing The Impacts Of Low Flows On Macrophytes And Benthic- Macro Invetebrate

Congress: 2015
Author(s): Jennifer Garbe (Edinburgh, UK), Lindsay Beevers, Gareth Pender
Heriot-Watt University1

Keyword(s): Sub-theme 10: Management of water resources,

Alteration of natural flow regimes caused by anthropogenic activities are affecting freshwater environments around the world (Malmqvist and Rundle 2002). In the UK, natural flow regimes are influenced by amongst others, land use change, water abstraction and transfer, hydroelectric power generation and flow impoundments. Water abstraction and transfers are a particular issue exacerbating low flows and exaggerating the effect of drought. These issues are created by management decisions and can therefore be altered.

The importance of freshwater environments span to all areas from rural to cities, and all species; humans, animals and plants. Fish for example provide food, transport of nutrients, recreational activities whereas macrophytes provide refugia for other species and help regulate flows. Benthic macro-invertebrate provide food for other species and help regulate and stabilise bottom substrates. These ecosystem services provided by the freshwater environment are all apparent on the SSSI designated river Nar in Norfolk. Despite its designation as a SSSI, over abstraction and low flows are causing stresses to these vital ecosystem services. This study seeks to address exactly how low flows are affecting two indicator species: Macrophytes (Ranunculus) and Benthic macro-invertebrates (Mayfly) in order to inform management decisions.


A Habitat model using CASiMiR 1D (Schneider et al., 2010) was built to demonstrate how various flows affect the indicator species at two sites in the river Nar. A 31 year period from 1980 to 2011 was modelled. CASiMiR requires three main inputs: firstly geometry data which was devised from cross sections taken of the river in May 2013 and further interpolated cross sections at the three sites. Secondly water surface profile data which was determined from a 1D hydraulic model (ISIS) built and calibrated based on field data. The full range of gauged flows at the sites were run through the model to determine water levels at each flow. Finally CASiMiR required habitat suitability data, for this study fuzzy rules were taken from CASiMiR which are based on ecological knowledge of the species. As these were determined for one river, the fuzzy rules and sets were calibrated using collected macrophyte data, collected benthos (kick sample) data and literature, slight changes were made.

Flow data from the Marham gauge was classified into low/high flow years. Flows below Q90 were used as the threshold below which 'low flows' occur and Q10 was used as a threshold above which 'high flows' occur. The same method was used on a seasonal basis to show if there were wet/dry antecedent conditions. Then using Suitability indices (SI) the habitat results were classified according to best to worst habitats on a five point (Likert) scale (table 1). Finally a summary was formed to show how during each year, percentage of good and bad habitat changes, and therefore how percentage habitat is affected during drought years.

Results and Discussion

Results for Ranunculus (graphs shown in paper) demonstrate how habitat availability changes in the wettest year (from 1980 to 2011) as 2001 and the driest year as 1991. For both sites the wet year is more suitable for Ranunculus than the dry year.

At site 2 the wet year has more suitable habitat availability. 72% of the areas were deemed either 'highly suitable' or 'suitable' in the wet year with only 30% deemed suitable in the dry year. This immediately shows that lower flows provide poorer conditions for Ranunculus. At site 3, the dry year has more area deemed 'highly unsuitable' and 'unsuitable' than the wet year. Likewise the dry year has less area at 'highly suitable' than the wet year. Site 3 had a less clear trend than site 2 as the area at 'suitable' which shows the dry year to have slightly more (5%) suitable area than the wet year. Ultimately however the wet year is better at this site for Ranunculus.

This said however, the results demonstrate that even under the driest conditions, Ranunculus do still have some available areas, and therefore can still survive under the driest conditions. This indicates over-abstraction causing low flows may not have a fatal effect on Ranunculus however they would still suffer. Furthermore the natural flow regime highlights a great importance to freshwater species and decreasing flows to abnormal levels according to the results would have a great impact on the species. The results demonstrate a need for further research as to what flows would be fatal to the species.

Antecedent conditions have a great influence on Ranunculus, in 2001 there had been 4 previous years in extremely wet conditions, in 1991 there were 2 preceding years in extremely dry conditions. It is clear therefore that wetter conditions provide more suitable habitat for Ranunculus, this response is not taken into account by models.

Similar results are presented for benthic macro-invertebrate where higher flows provide better habitat availability. This corresponds to the collected LIFE scores for macro-invertebrate.


The result presented here, combined with the benthic macro-invertebrate results show higher flows are important for Ranunculus, the velocity is particularly important for the species (Cranston and Darby 2004) reduced flows create reduced velocities and therefore less available habitat. The subsequent years after dry periods would cause huge degradation to the species due to little habitat availability. Management decisions can be aided by using these habitat models to demonstrate how affecting flows by water abstraction could impact upon the habitats within the river. The research will be further extended to be modelled in 2D and to show how trading affects these habitats. Cranston, E. and E. Darby (2004). 'Ranunculus in Chalk Rivers Phase 2'

Malmqvist, B. and S. Rundle (2002). 'Threats to the running water ecosystems of the world.' Environmental Conservation. Vol 29. (2) P: 134-153.

Schneider, M., M. Noack, T. Gebler and I. Kopecki (2010). 'Handbook for the habitat simulation model CASiMiR'

2011 IWRA - International Water Resources Association - - Admin